The invention relates to a method and apparatus for compensating synchronization errors in the transport of a record carrier in non-impact matrix printers, notably electrophotographic printers, in which the stored information of a line of print is formed by picture lines which are to be successively recorded with constant time intervals, the spacing of the picture lines being dependent on the transport speed of the record carrier and the recording frequency of the picture lines.
Non-impact printers do not involve direct contact between the printing members and the record carrier. Instead a line of print is recorded by spraying (for example, ink jet printers) or forming a latent image (electrostatic and electrophotographic printers) which is subsequently developed and fixed. Moreover, the characters of a line of print can be formed in various ways. For example, when a single character is composed from 5.times.7 dots, 5.times.7 individually activated recording elements can be provided. In that case, the character is composed in one operation. It is alternatively possible to arrange only five recording elements over one another and to make these elements perform seven steps in order to compose the single character. In both cases, a line of print is formed one character after the other. A third possibility consists in that there is provided only a single recording element or an array of adjacently arranged recording elements, so that for the formation of the characters of a line of print the relevant dots of a picture line are formed successively or simultaneously and five picture lines are successively recorded. This possibility is preferably used in line printers. This method is used notably in electrostatic and electrophotographic printer and ink jet printers. In electrostatic printers, preferably as many electrodes are adjacently arranged as dots can be contained in a picture line. The necessary picture lines are then successively recorded by transporting the record carrier. Instead of the multitude of electrodes, electrophotographic printers utilize only a single recording beam which is horizontally deflected from dot to dot within a picture line. It is thus also possible to record the same content of a picture line several times. A picture line then consists of several sub-lines.
The invention relates to such printers in which the lines of print are recorded one picture line after the other. The recording speed for a line of print depends on the transport speed of the record carrier and on the speed of actuation of the recording elements in electrostatic printers or on the deflection speed (line frequency) in electrophotographic printers. The quality of the printed picture itself depends mainly on the spacing of the dots. The more these dots overlap, the better the printed picture will be. The dots in the picture line can be made to overlap in electrostatic printers by arranging the electrodes in a configuration with several rows of electrodes which are staggered with respect to one another. In electro-optical printers, a picture line is formed by horizontal deflection of the recording beam. The total deflection corresponds to the feasible length of a line of print. The overlapping of the dots in the vertical direction is not problematic, because the spacing between the picture lines can be chosen at random and is dependent on the transport speed of the record carrier. A line of print can thus be readily composed of from 10 to 100 picture lines. The spacing between the picture lines is customarily from 0.05 to 0.2 mm. A picture is thus obtained which is substantially equivalent to a conventional printed picture.
A condition for such a quality of the printed picture, however, is that the fixed spacing between the picture lines must remain constant as much as possible over long periods of time. Even though the synchronization of the recording of the picture lines by the electronic control of the recording elements or the line frequency of the recording beam can be readily achieved, problems arise as regards the transport of the record carrier; these problems are caused mainly by the drive elements, for example, transmissions, motor, etc.
Synchronization errors in the transport speed of the record carrier can be tolerated during the recording of a single line of print if they do not exceed a few percents . For the printing of larger sheets, for example, DIN A4, with a multitude of lines of print, however, the synchronization errors must be substantially smaller, because errors in the spacing of the picture lines due to transport speed fluctuations can accumulate over the full length of the sheet. Therefore, the tolerance for such long-term errors should be from 0.2 to 0.5%. However, this can be achieved only with great difficulty in the record carrier drive, for example, by accurately controlled motors. The less expensive asynchronous motors have a transport speed tolerance of a few percents, taking into account line voltage fluctuations and different operating modes of the machine, e.g. intermittent or continuous printing.